In the early days of the Universe, the Guardians declared themselves the protectors of the cosmos and enemies of evil. To enforce their will and guard against alien menaces of all sorts, the Guardians had created a legion of robotic sentinels called the Manhunters. But the Manhunters had come to resent their servitude and the rebelled against the Guardians, eventually breaking away and forming their own robotic society where they pursuied their own agenda (which often included interfering and foiling the plans of the Guardians of the Universe).
Chastened by the failure of the Manhunters, the Guardians decided that their newest force of soldiers for good would consist of living beings, ones who had free will and strong moral character. To arm this new legion of celestian knights, the Guardians created the Power Rings, rings of inconceivably advanced technology that allowed their wearers to project green beams of energy that had the power to do literally anything... provided the one wielding the ring had the force of will to achieve his goals. The Power Rings were invincible against anything except objects of the color yellow. The Guardians often stated that the Rings' weakness against yellow objects was a "necessary impurity," though scholars (and comic book fans) often speculated that the "impurity" was actually designed as a mental safeguard, to prevent any member of the Green Lantern Corps from thinking himself or herself (or itself) invincible and above the rule of law.
The Corps was a volunteer organization of 3600 Green Lanterns with each assigned to an individually numbered assigned sector of space such as Hal Jordan's which was Sector 2814. Heavily populated Sectors like 2814, also required a backup Lantern who was expected to ready to be called to duty in the regular Lantern's absence on short notice. Each member had a great deal of autonomy as to their methods in their jurisdiction, subject to review by the Guardians if they felt the Green Lantern in question has abused his/her assigned authority. Also, the individual Lanterns were responsible for arranging their replacements if they are near retirement or death. If the Lantern died before that obligation was met, the Guardians would personally go out into the field to recruit a replacement.
The Green Lantern Corps was obviously modeled after the Lensman science fiction series of E.E. "Doc" Smith. There were many parallels between the two: both were intergalactic law enforcement agents chosen for their high moral character and willingness to do good, and empowered with an awesome weapon of alien design that also had safeguards against abuse and evil trickery.
The Green Lantern Corps patrolled the D.C. Universe for over three billion years. In that vast length of time, only two Green Lanterns rebelled and fell into evil: Sinestro, the rogue Green Lantern, and Hal Jordan, angered over the loss of his city Coast City and the heartlessness of the Guardians.
Comic book writers were attracted to the novelty of the Green Lantern Corps. An occasional backup series in the Green Lantern comic book series, Tales of the Green Lantern Corps, featured guest writers who often penned short stories about the idiosyncracies of a various alien Green Lantern, who might never be heard of again.